There are known a number of automobile engines which include: a valve operating mechanism for selectively operating low or high speed cams and activating valves by the selected cams at proper timings so as to increase an engine output; a valve operating mechanism for selectively operating low or high speed cams, activating valves by the selected cams, and allowing a partial cylinder operation by disconnecting a part of engine cylinders; or a valve operating mechanism for suspending or suppressing the operation of a part of intake and exhaust valves depending upon operating conditions of the engine so as to increase an engine output.
A control device sets various operating modes for controlling such a valve operating mechanism. For instance, when the partial cylinder operation mode is selected, the control device controls the valve operating mechanism having the cylinder disconnecting function in a manner such that the valve operating mechanism suspends the operation of intake and exhaust valves of cylinders to be disconnected and the fuel supply to the disconnected cylinders. After the partial cylinder operation mode is canceled, the control device allows the valve operating mechanism to activate all the cylinders and permits the fuel supply to them. During the full cylinder operation mode at a low engine speed, the control device controls the valve operating mechanism in a manner such that low speed cams operate the intake and exhaust valves so as to increase volume efficiency. Conversely, during the full cylinder operation mode at a high engine speed, high speed cams operate the intake and exhaust valves so as to increase the volume efficiency.
Usually, a speed at which fuel supply is suspended (called "fuel supply suspending speed" hereinafter) is set in internal combustion engines so as to prevent an overspeed of the engine. For instance, Japanese Patent Laid-open Publication No. Hei 2-161,154 describes a system in which a fuel supply suspending speed is set relatively low when the valves are operated by cams which push the valves at a high speed, while such a speed is relatively high when the valves are operated by cams which push the valves at a low speed.
In conventional control devices, the fuel supply suspending speeds are determined based only on the speed at which the valves are pushed by the cams. Therefore, if operation mass increases in a valve system which is operated by cams having the same profile, a valve spring fails to follow, which causes unfavorable valve bounce. This phenomenon would result in trouble such as valve breakage and excessive friction. To overcome this trouble, it is advantageous to strengthen the valve spring and to reduce the operation mass of the valve system. Such measures, however, would lead to new problems such as an increased friction at contact areas between the cams and rocker arms, or insufficient strength of the valve operating mechanism.
It is therefore an object of the invention to provide a device and a method for protecting the internal combustion engine against damages in the valve system when changing a speed to operate the valves.